Sup35p is a Saccharomyces cerevisiae protein with an essential function in promoting translation termination. In a state referred to as [PSI], Sup35p adopts an altered conformation causing it to aggregate. [PSI] is believed to be propagated as an altered form of Sup35p in a manner consistent with the prion model. [PSI] is maintained cytoplasmically and is inherited infallibly during mitosis and meiosis. By an unknown mechanism, [PSI] is eliminated from yeast cells during growth in the presence of millimolar levels of guanidine-hydrochloride (GuHCl). Our goal is to elucidate the underlying mechanisms involved in prion metabolism using [PSI] as our model system. Our approach is to isolate mutant strains displaying aberrant [PSI] behavior, identify the genes involved, and determine the mechanisms behind the effects. We have discovered that expression of proteins involved in response to environmental stress are aberrantly expressed in a mutant strain in which [PSI] has become mitotically unstable. For two among four additional mutant strains in which [PSI] has become weakened or unstable, the wild type alleles have been identified. To date, attempts at isolating the wild type allele of a mutation allowing maintenance of [PSI] despite the presence of GuHCl in the growth medium has resulted in isolation of an unlinked suppressor. We have found a known mutant to be strikingly hypersensitive to the curative effects of GuHCl. I have also discovered that certain combinations of known mutations lead to instability of [PSI] during meiosis. This has been found to reside in defects during the process of meiosis rather than in the products of it. I have also identified mutants in which [PSI] is progressively eliminated from spores as they age. In the course of the [PSI] studies I have discovered that a previously known mutation causes significant read-through of translation termination independently of [PSI]. Such an effect for this mutant has yet to be described. This project is not a major goal of the laboratory but will be pursued until the results are published. - Prion, [PSI], Sup35, Yeast